using System;

using System.Security.Cryptography;

using System.IO;

                    // https://stackoverflow.com/questions/68673772/how-to-decrypt-string-from-openssl-encrypt-in-c-sharp

//https://stackoverflow.com/questions/19719294/decrypt-string-in-c-sharp-that-was-encrypted-with-php-openssl-encrypt

// https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rijndaelmanaged?view=net-5.0

public class Program

{

    private static string key = "12345678901234567890123456789012";

    private static string originalText = "does this work";

    private static string encryptedValue = "a64d63874ba9ee8f5a5028cb40ab70a4yFFQwLuOHeWouyfp0dyrnw==";

    public static void Main()

    {

        // Console.WriteLine(OpenSslDecrypt(encryptedValue, key));

        Console.WriteLine(DecryptPHP(encryptedValue));

    }

    public static string DecryptPHP(string encrypted)

    {

​

        //var _key = System.Text.Encoding.UTF8.GetBytes(key);

        //var _iv =  System.Text.Encoding.UTF8.GetBytes(iv);

        // var _key = FromHex(key);

        var _iv = new byte[16];

​

        var salt = encryptedValue.Substring(0, 32);

        var unsaltedEncryptedValue = encryptedValue.Substring(32);

        Console.WriteLine("salt: " + salt);

        Console.WriteLine("unsaltedEncryptedValue: " + unsaltedEncryptedValue);

​

        //get the encrypted data and decrypt

        byte[] encryptedBytes = Convert.FromBase64String(unsaltedEncryptedValue);

        byte[] unsaltedEncryptedBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(unsaltedEncryptedValue);

        byte[] keyBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(key);

        byte[] ivBytes = System.Text.ASCIIEncoding.UTF8.GetBytes(salt);

        return DecryptStringFromBytesAes(unsaltedEncryptedBytes, keyBytes, ivBytes);

    }

    /*

    public static string OpenSslDecrypt(string encrypted, string passphrase)

    {

        // base 64 decode

        byte[] encryptedBytesWithSalt = Convert.FromBase64String(encrypted);

        // extract salt (first 16 bytes of encrypted)

        byte[] salt = new byte[16];

        byte[] encryptedBytes = new byte[encryptedBytesWithSalt.Length - salt.Length]; // why -8

        Buffer.BlockCopy(encryptedBytesWithSalt, 16, salt, 0, salt.Length);

        // get key and iv

        // byte[] key, iv;

        // DeriveKeyAndIV(passphrase, salt, out key, out iv);

        return DecryptStringFromBytesAes(encryptedBytes, key, iv);

    }*/

​

    public static string DecryptStringFromBytesAes(byte[] cipherText, byte[] key, byte[] iv)

    {

        // Check arguments.

        if (cipherText == null || cipherText.Length <= 0)

            throw new ArgumentNullException("cipherText");

        if (key == null || key.Length <= 0)

            throw new ArgumentNullException("key");

        if (iv == null || iv.Length <= 0)

            throw new ArgumentNullException("iv");

​

        // Declare the RijndaelManaged object

        // used to decrypt the data.

        RijndaelManaged aesAlg = null;

​

        // Declare the string used to hold

        // the decrypted text.

        string plaintext;

​

        // Create a RijndaelManaged object

        // with the specified key and IV.

        aesAlg = new RijndaelManaged { Mode = CipherMode.CBC, Padding = PaddingMode.Zeros, KeySize = 256, BlockSize = 128, Key = key, IV = iv };

​

        // Create a decrytor to perform the stream transform.

        ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

        // Create the streams used for decryption.

        using (MemoryStream msDecrypt = new MemoryStream(cipherText))

        {

            using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))

            {

                using (StreamReader srDecrypt = new StreamReader(csDecrypt))

                {

                    // Read the decrypted bytes from the decrypting stream

                    // and place them in a string.

                    plaintext = srDecrypt.ReadToEnd();

                    srDecrypt.Close();

                }

            }

        }

​

        return plaintext;

    }

}